The fires have stretched across South Wales, with most happening between Bridgend and Torfaen.

The grass fires are a problem which have affected South Wales more so than anywhere else in the country.

A Welsh Government report shows evidence that during periods of recession the incidence of some types of deliberate fire-setting increases.

Of the grassland fires in 2013-14, 56% were in South Wales, 32% were in Mid and West Wales and 12% were in North Wales.

Cerith Griffiths, secretary of the FBU in Wales, said the fires had a huge impact on crews and their resources.

“These fires not only tie up immediate resources but there is a huge cost implication.

“As we have seen budgets massively squeezed, fire stations closed, the downgrading of certain stations and the removal fire engines, this extra pressure is never good."

Warning: At least one fire has come dangerously close to homes (Image: Wales News Service)

Mr Griffiths, who has worked in the fire service for 20 years, said grass fires put firefighters under huge stress.

“We have much fewer resources than we used to have.

"These kinds of incidents will tie crews up for hours on end and means they are isolated from getting to other serious incidents such as property fire or a road traffic collision.

"It takes us longer to respond, putting crews under even more pressure.”

Andrew Schofield, regional land manager for Natural Resources Wales, said wild fires can have serious repercussions for local communities, wildlife and protected sites.

“These fires result in the destruction of nesting birds and other rare wildlife and plants that make up the forest ecosystem.

"There is also the negative image left by vast areas of the beautiful south Wales landscape left scorched by these fires, which has a damaging economic impact on the area’s businesses and tourism.

Burning issue: Children as young as 11 have been arrested after a spate of fires (Image: Wales News Service)

“Where wildfire occurs on land which we manage we support the fire and rescue services across Wales by providing them with crucial information about the makeup of the forests or sites, potential risk areas, and the best access points for them to tackle the blazes.”

Watch manager Tony Redman, who heads up the Community and Safety Parnership forSouth Wales Fire and Rescue Service, said bringing those involved to justice was very unlikely and prevention was the only effective way to tackle the problem.

“We were up at one grass fire in Maerdy at the weekend and there were three 14-year-olds on the mountain, one of whom had a can of lager.

"Asked what they were doing there they said they were looking for a mobile phone.

“There was no evidence to connect them with the fire.

"Unfortunately most of the time we are left powerless because it’s hard to connect a person with a fire such as this.

He said Northumberland Fire Service, which manages a similar terrain to that in south Wales, had only experienced two grass fires so far this spring.

“For us this is a cultural issue and the figures bear this out.

"We have high unemployment and high levels of deprivation and that is certainly a key factor here.

"One of the main ways for a fire to spread is on ground that slopes upwards.

"A fire in flat ground will not spread as quickly.

"You might only have a metre-wide fire but within 10 minutes that could have spread to two acres.”

Hot spot: Fire crews had to put out a grass fire in Tylorstown (Image: Media Wales)

Mr Redman, who was tackling a 100-hectare grass fire at Maerdy Mountain with his colleagues on Sunday, said: “We worked for six hours solid against flames that were six-to-eight metres high and were soaking to the bone.

“One of the firefighters said he was shaking and had never been that angry. We couldn’t get anyone to relieve us.

"We had fire engines from Cardiff and Newport and up in Ferndale, which is 18 miles away.

"They were dragged in because the resources were so stretched. The guys on the ground are absolutely shattered.”